Method of cladding steel with titanium or zirconium



Unite ee Patent METHOD OF CLADDING STEEL WITH TITANIUM 0R ZIRCONIUM 2 Claims. (Cl. 29-4703) This invention relates to the cladding of steel with a corrosion-resistant metal. More particularly it relates to the cladding of steel with a layer of titanium or zirconium. V l

The exceptional resistance of titanium and zirconium to corrosion when exposed to corrosive liquids and atmospheres is well known. Titanium, for example, is in a class with platinum in resisting attack by marine atmospheres, and also has outstanding resistance to attack by many acids. Zirconium is also extremely inert to a variety of strong acids and corrosive atmospheres. The corrosion-resistant properties of titanium and zirconium make these metals very desirable for use as structural materials in many commercial applications. At the present time, however, these metals are relativelyexpensive and are in short supply, factors which prohibit their extensive use for general structural purposes.

A known expedient for utilizing the corrosion-resistant properties of high cost materials such as titanium and zirconium comprises cladding a less costly base metal with a layer of the corrosion-resistant material to form a composite bi-metallic material that can be fabricated into desired structural shapes and apparatus. Accordingly it has been proposed that a structural base metal, such as mild steel, be clad with a layer of titanium or zirconium in order to take advantage of both the low cost and high strength of the steel base metal and the exceptional corrosion-resistant properties of the cladding metal. 7

Various procedures for cladding mild steel plate with a layer of titanium or zirconium have been suggested, and one of the most interesting of these involves the diffusion-bonding of the cladding layer to the base metal by means of a high temperature pressure welding operation. In this operation, the steel base metal and the cladding layer of titanium or zirconium are brought intimately together with great force and at relatively high temperatures to obtain a solid state diffusion bond between the steel and the titanium or zirconium. Commonly the necessary-intimate contact between the two metal layers is accomplished by passing the assembled layers through a strand of rolls capable of exerting the relatively great pressure required. Although titanium and zirconium will form a diffusion bond with mild steel under these conditions, it has been found that the bond is imperfect and is subject to rupture due to the formation of oxide films on the surfaces ofthe. metals and especially to the formation of brittle alloys of the two metals at the interface of the metals. The brittle inter-metallic compounds which form at the interface of the two metals are inherent in both the iron-titanium and the iron-zirconium alloy systems. The formation of these brittle intermetallic compounds cause the bond between the iron and the cladding metal to be weak so that the resulting composite metal plate is not amenable to the usual forming processes. 4

2,908,969 Patented Oct. 20,1959

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I have now found it possibleto clad mild steel plate with a layer of titanium or zirconium while avoiding the formation of the undesirable iron-titanium or ironzirconium inner metallic compounds. Pursuant to my discovery, the layer of corrosion-resistant metal is firmly bonded to the steel base metal through the agency of an intermediate layer of a bonding metal selected from the group consisting of chromium, cobalt and molybdenum. The clad steel product of the invention is produced by forming a composite of a layer of steel base metal, an intermediate layer of one of the bonding metals chromium, cobalt or molydedum, and a cladding layer of one of the corrosion-resistant metals titanium and zirconium. The metal composite is heated to a temperature of at least 1400 F. in an inert atmosphere and the several layers of the composite are compressed together while at this. temperature to. effect a reduction in the thickness of the composite of about one-half of its initial thickness.

The method of my invention results in the formation of a composite clad steel product in which the steel base metal is firmly bonded to a layer of corrosion-resistant titanium or zirconium. The steel base metal may comprise any mild steel plate suitable for use in the fabrication of the corrosion-resistant equipment ultimately and thinner plates may be used. The cladding layer should be of commercial purity, that is it should be free from embrittling impurities and from any alloying metal which would impair the usefulness and the corrosion resistance of the compositemetal product.

; The layer of bonding metal disposed between the steel base metal and the corrosion resisting sheet of titanium or zirconium pursuant to my invention prevents the formation of undesirable iron-titanium and iron-zirconiumcompounds. The materials which I have found useful for this purpose are, as hereinbefore stated, chromium, cobalt and molybdenum. The chromium and cobalt are most conveniently incorporated in the composite metal product by electro-plating these metals on the surface of the cladding layer of titanium or zirconium. The

" chromium and cobalt are plated on the surface of the cladding metal from aqueous solutions of salts of cobalt or chromium by means of conventional electro-plating procedures. Molybdenum, on the other hand, can be readily fabricated into a thin sheet and is advantageously incorporated in the composite metal assembly in this form. Although chromium will form a satisfactory bond with mild steel under the conditions employed in my process, I prefer, after electroplating the titanium with a layer of chromium, to electrodeposit on the chromium a supplementary layer of nickel which forms a diffusion bond with the mild steel base metal somewhat more readily thandoes chromium. The thickness of the intermediate bonding metal disposed between the advantageously within the range of about 0.5 to 5 mil,

although bonding layers outside this thickness range may be used effectively in some instances.

The pressure Welding of the corrosion-resistant metal and the intermediate layer of bonding metal to the steel base. metal must be carried out at. moderately high temperatures. A temperature of at least about 1400 F. is required for efficient pressure welding of the several components of. the composite. metal. assembly- For optimum bonding of chromium and nickel-plated chromium to themild base plate, I presently prefer to use a temperature within the. range of about 1600" F. to 1800 F. For cobalt, I- prefer to use a temperature within the range. of 1.40.0 to 1500' F. and for molybdenum atemperature within the range of 1800 to 2000 F. It must be understood, however, that these preferred. ranges are for temperatures which lead to most eflicient operation and that other temperatures, provided. they are at least as high as about 1.400 F., are eifective in practicing the invention.

The bonding of the steel base metal to the layer. of bonding metal at the aforementioned elevated temperatures pursuant to my invention must be carried out under conditions which avoid the formation of a surface oxide film. that. otherwise would form on the steel base metal when it is heated in the presence of the ambient oxidizing atmosphere. I have found that these conditions can. be satisfied by transforming the metal assembly into a gas-tight casing. and' establishing within the resulting gastight casing the necessary inert atmosphere so that conventibnal rolling equipment can be employed to effect the pressure-welding operation.

To form the aforesaid gas-tight casing, a mild steel base metal plate, an intermediate layer of bonding metal, a cladding layer of titanium or zirconium, and a temporary covering layer of mild steel or similar material are assembled to form a composite metal sandwich. To prevent the temporary covering layer from becoming bonded to the adjoining corrosion-resistant metal, the ad'- jacent, surface of the titanium or zirconium is advantageously coated with a thin film of colloidal graphite. Strips of mild steel or the like are then welded. to the proximate edges of the steel base layer and the temporary steel covering layer of the composite assembly to form the gas-tight casing enclosing the corrosion resistant metal. All air is withdrawn from the inside of the casing thus formed and is replaced with an inert gas such as argon. The argon-filled casing is then heated to the aforementioned bonding temperature and the casing is passed between conventional reducing rolls so as to reduce the thickness of the composite assembly by about 50%. On completion of the rolling operation, the edge strips welded to the base plate and to the temporary covering plate are cut away. The covering plate is then removed from the composite assembly exposing the corrosion resistant layer of the composite metal assembly. The resulting assembly comprises the desired titaniumor zirconium-clad base metal.

The following examples are illustrative, but not limitative, of the practice of my invention:

Example I A piece of commercially pure titanium 2' x 2 x inches was plated with cobalt toa thickness of about onemil. The plating bath consisting of an aqueous solution of cobaltous sulphate and the electrolysis was carried out at a temperature of 100 F. at acurrent density of 4 amps/dm One surface of' the plated titanium was coated .with a. film of colloidal graphite and the'titanium was placed between two clean pieces of mild steel 3 x 3 x inches. Spacers of mild steel strip were placed between the two steel plates adjacent the edges ofthe tit-anium and the corresponding edges of the steel plates. The steel strips were then welded totheproximate edges of the two steel plates thus forming a gas-tight casing from. the composite assembly. A small hole communicating-.with: theinterior of the casing. was formed in. the

weld and spacer strip and a inch diameter pipe was Welded to the edge of the casing at this hole.

The pipe was connected to a vacuum pump and the space within the casing was evacuated to a pressure of about 20 microns. The interior of the casing was then filled with dry argon gas. The; assembly was heated to 1450 F. and. reduced by rolling to about 50% of its original thickness. On. completion of the rolling operation the continuous weld connecting the two steel plates was cut away and the temporarycover plate was removed. The resulting product comprised a steel base plate to which was firmly bonded a layer of titanium less than v inch thick. The colloidal graphite used to prevent bonding of the titanium to the steel cover plate had no observable effect on the titanium at the rolling temperatures employed. The titanium layer was firmly bonded to the steel base metal so that the composite metal plate could be subjected to severe working without evident separation of the'layers of the composite.

Example 11 A piece: of. commercially pure titanium was plated with chromium electrodeposited thereon from an aqueous solution.. One surfaceofthe chromium plated titanium sheet was then assembled with a base plate of mild steel, a similar covering plate of mild steel, and steel spacer strips to form a gas-tight casing in the manner described in Example- I. After establishing; an inert atmosphere within the gas-tight casing, the assembly Was rolled at a temperature of 1800 F. to: reduce the thickness of,

the assembly by about 50% The edge strips welded to the edges of the assembly Were then cut away and the covering plate removed to expose the corrosion resistant layer of the composite metal plate; The titanium was firmly bonded to the steel base metal and the composite assembly was subjected to severe working without appreciable separation of the layers of the assembly.

Example [11 A composite assembly comprising a mild steelbase plate, an intermediate bonding layer of molybdenum having a thickness of 2 mils, a sheet of titanium having a thickness of 4; inch, and a covering layer of mild steel was formed into a gas-tight casing by means of steel spacer strips Welded to the edges of the steel base plate and the steel covering plate. Arr. inert atmosphere was established withinthe casing and it was their rolled at a temperature of- 1800" F- to reduce the thickness. thereof by about 50%. The edge strips were cut away and the covering plate removed to expose a titanium clad steel plate having excellent working properties.

Example V V The procedures. outlined in each of the foregoing examples were repeated using a plate of zirconium in place of the titanium to form a zirconium-clad steel product having, in each case, excellent working properties.

As seen from the foregoing; description and examples, a corrosion-resistant metal-clad steel product can be formed pursuant to my inventionwhich retains the corrosion-resistant properties of the cladding metal while taking advantage of the low: cost: and high strength of" the steel base metal. The cladsteel productofmy invention incorporated in apparatus in which the desirable characteristics of the product are of importance.

I claim:

1. The method of cladding steel with a layer of a corsion-resistant metal selected from the group consisting of titanium and zirconium which comprises forming a composite of the steel base metal, an intermediate layer of a bonding metal selected from the group consisting of chromium, cobalt and molybdenum, a layer of the corrosion-resistant metal, and a temporary covering layer of steel, said temporary covering layer being separated from the layer of corrosion-resistant metal by a film of colloidal graphite, transforming said composite into a gas-tight casing by Welding steel strips to the proximate edges of the steel base metal and the temporory steel covering layer, evacuating the gas-tight casing to remove substantially all of the air from said gas-tight casing, thereafter admitting a dry inert gas to said gas-tight casing, whereby the formation of an oxide film on the surface of the steel within the casing during the heating step which follows is prevented and thereby establishing an inert atmosphere within said gas-tight casing formed from said composite, heating said composite to a temperature of at least 1400 F., compressing the layers of the composite together to effect a reduction in thickness of about one-half of the initial thickness of the composite, cutting away the steel edge strips welded to the steel base metal and the steel covering layer, and removing the temporary steel covering layer from the resulting corrosion-resistant metal-clad steel base metal.

2. The method of cladding steel with a layer of a corrosion-resistant metal selected from the group consistsing of titanium and zirconium which comprises depositing a layer of chromium on at least one surface of a layer of the corrosion-resistant metal, depositing a supplementary layer of nickel on said layer of chromium, forming a composite of a layer of steel base metal, said supplementary layer of nickel, said layer of chromium, said layer of the corrosion-resistant metal, and a temporary covering layer of steel, said temporary covering layer of steel being separated from the layer of corrosion-resistant metal by a film of colloidal graphite, transforming said composite into a gas-tight casing by afiixing steel strips to the proximate edges of the steel base metal layer and the temporary steel covering layer, evacuating the gastight casing to remove substantially all of the air from said gas-tight casing, thereafter admitting a dry inert gas to said gas-tight casing, whereby the formation of an oxide film on the surface of the steel within the casing during the heating step which follows is prevented and thereby establishing an inert atmosphere within said gastight casing formed from said composite, heating said composite to a temperature of at least 1400 F., compressing the layers of the composite together to effect a reduction in thickness of about one-half of the initial thickness of the composite, cutting away the steel stripswelded to the steel base metal and the steel covering layer, and removing the temporary steel covering layer from the resulting corrosion-resistant metal-clad steel base metal.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES WADC Report 52-313, part I, published Nov. 1952 by Lewis et a1. Wright Air Development Center. 

1. THE METHOD OF CLADDING STEEL WITH A LAYER OF A CORSION-RESISTANT METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM AND ZORCONIUM WHICH COMPRISES FORMING A COMPOSITE OF THE STEEL BASE METAL, AN INTERMEDIATE LAYER OF A BONDING METAL SELECTED FROM THE GROUP CONSISTING OF CHROMIUM, COBALT AND MOLYBDENUM, A LAYER OF THE CORROSION-RESISTANT METAL, AND A TEMPORATY COVERING LAYER OF STEEL, SAID TEMPORATY COVERING LAYER BEING SEPARATED FROM THE LAYER OF CORROSION-RESISTANT METAL BY A FILM OF COLLOIDAL GRAPHITE, TRANSFORMING SAID COMPOSITE INTO A GAS-TIGHT CASING BY WELDING STEEL STRIPS TO THE PROXIMATE EDGES OF THE STEEL BASE METAL AND THE TEMPORARY STEEL COVERING LAYER, EVACULATING THE GAS-TIGHT CASING TO REMOVE SUBSTANTIALLY ALL OF THE AIR FROM SAID GAS-TIGH CASING THEREAFTER ADMITTING A DRY INERT GAS TO SAID GAS-TIGH CASING, WHEREBY THE FORMATION OF AN OXIDE FILM ON THE SURFACE OF THE STEEL WITHIN THE CASING DURING THE HEATING STEP WHICH FOLLOWS IS PREVENTED AND THEREBY ESTABLISHING AN INERT ATMOSPHERIC WITHIN SAID GAS-TIGHT CASING FORMED FROM SAID COMPOSITE, HEATING SAID COMPOSITE TO A TEMPERATURE OF AT LEAST 1400*F., COMPRESSING THE LAYERS OF THE COMPOSITE TOGETHER TO EFFECT A REDUCTION IN THICKNESS OF ABOUT ONE-HALF OF THE INITIAL THICKNESS OF THE COMPOSITE CUTTING AWAY THE STEEL COVERING LAYER, AND REMOVING THE METAL AND THE STEEL COVERING LAYER, AND REMOVING THE TEMPORARY STEEL COVERING LAYER FROM THE RESULTING CORROSION-RESISTANT METAL-CLAD STEEL BASE METAL. 